cmd/compile: incorrectly reports x - minimumValue overflows for integers

[Jorropo]

Go version

go version devel go1.24-0fe775e9f3 Mon Jul 29 05:12:05 2024 +0000 X:rangefunc linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/tmp/go-build'
GOENV='/home/hugo/.config/go/env'
GOEXE=''
GOEXPERIMENT='rangefunc'
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/hugo/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/hugo/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/hugo/k/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/home/hugo/k/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='devel go1.24-0fe775e9f3 Mon Jul 29 05:12:05 2024 +0000 X:rangefunc'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/hugo/.config/go/telemetry'
GCCGO='/usr/bin/gccgo'
GOAMD64='v3'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/hugo/k/OpenAirways/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build671918439=/tmp/go-build -gno-record-gcc-switches'

What did you do?

compile the following code:

const oneTurn = 1 << 16

type Rot16 uint16

func (x Rot16) Alignement(y Rot16) (alignement int16, reversed bool) {
	alignement = int16(x - y)
	reversed = abs(alignement) > abs(alignement-oneTurn/2)
	if reversed {
		alignement -= oneTurn / 2
	}
	return
}

func abs[T ~int | ~int8 | ~int16 | ~int32 | ~int64 | ~float32 | ~float64](a T) T {
	if a < 0 {
		a = -a
	}
	return a
}

What did you see happen?

./prog.go:9:46: oneTurn / 2 (untyped int constant 32768) overflows int16
./prog.go:11:17: oneTurn / 2 (untyped int constant 32768) overflows int16

What did you expect to see?

Nothing (compiles successfully).

[Jorropo]

I use a+-b as a workaround.

This might be working as expected as far as the go spec goes, but then I think the language should be changed, reading a+-b feels wrong.

[gabyhelp]

Related Issues and Documentation

• cmd/compile: out of range integer in range loop crashes the compiler #65133 (closed)
• Signed non-negative divide by power of 2. #44530 (closed)
• cmd/compile: range over rune literal causes internal compiler error #64471 (closed)
• cmd/compile: constant overflows when assigned to package level var (Go 1.20 regression) #58293 (closed)
• cmd/compile: bad code generation for ARM (bounds test fails) #41780 (closed)
• go/types: tip: math/all_test.go:1963:2: constant overflow #45874 (closed)
• cmd/compile: inconsistent integer arithmetic result on Go 1.22+arm64 with/without -race #68227 (closed)
• cmd/compile: ICE in SSA when make is invoked with negative value instead of runtime panic about "len out of range" #36259 (closed)
• cmd/compile: miscompilation of simple arithmetic program targeting darwin/arm64 #62723 (closed)
• cmd/compile: corrupted value #20530 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[mknyszek]

CC @golang/compiler

[cherrymui]

As you mentioned, this is working as intended. It is not reporting the result of the subtraction overflows, but the conversion of untyped int 32768 to int16 (which happens before the subtraction) overflows.

a+-b feels unnatural, but for int16, when b=32768, a+b and a-b are actually same. You may as well just write a+b.

[griesemer]

Working as expected as @cherrymui already explained. The - is crucial because it brings 32768 (which is not a valid int16 number) to -32768 (which is a valid int16 number) so the conversion can succeed.

This is just a typical edge case of fixed-width 2's complement arithmetic that one has to be aware of. We're not going to change the language for that. For one, making adjustments in the spec to comprehensively cover such cases would amount to a lot of complexity for very little benefit.

Closing as working as intended.